The present invention relates to a method of growing large diameter piezoelectric Ln3Ga5.5Me0.5O14 (Ln=La, Pr, Nd and Me=Nb,Ta) and Ln3Ga5Mxe2x80x2O14 (Ln=La, Pr, Nd Mxe2x80x2=Si, Ti, Zr, Hf) single crystals and solid solutions on this basis and more particularly to a method of growing such crystals for use as wafers in bulk acoustic wave (BAW), surface acoustic wave (SAW), and pseudo surface acoustic wave (PSAW) devices, having an excellent temperature characteristic and a large electromechanical coupling factor. The SAW devices are currently used, for example, as bandpass filters, resonators, delay lines and converters, in a broad range of wireless applications, cellular communication devices and cable TV.
Piezoelectric material based on lanthanide gallium crystals, including the langasite family of crystals, i.e. La3Ga3SiO14, referred to as langasite (LGS), La3Ga5.5Nb0.5O14, referred to as langanite (LGN), and langatate La3Ga5.5Ta0.5O14 (LGT) are known to be useful for piezoelectric applications. A SAW device having a LGS single crystal substrate is disclosed in U.S. Pat. No. 5,821,673, U.K. Pat. No 2,328,815, RU Pat. No 2073952, and U.S. Pat. No. 5,917,265. A LGT single crystal substrate having a prescribed range of Euler angles for substrate and crystal orientation to improve signal processing in a SAWdevice, is disclosed in U.S. Pat. No. 6,097,131.
A SAW device comprising a wafer constructed of a trigonal langasite crystal cut at predominated cut angles is disclosed in U.S. Pat. No. 5,981,673. An optimal cut for SAW devices made from langatate crystals is disclosed in U.S. Pat. No. 6,097,131. A substrate for piezoelectric device and SAW devices composed of a single crystal of langanite is proposed in JP Pat. No 11106294A.
The method of oriented crystallization from a melt, eliminating any contact with the side faces of a growing crystal with solid walls, are now well known and increasingly becoming widespread. These are the Czochralski (conventional Czochralski), Stepanov, Verneuil and floating zone techniques, characterized by a fixed orientation of crystallization at a fixed position of the solidxe2x80x94liquid interface. Each of these techniques has a predominant field of application with the common feature of the elimination of contact between the crystal and a solid wall. As a result, both the shape and the size of a growing crystal are essentially determined by capillary forces, which form a meniscus in the interface boundary zone. In addition, the crystallization process also depends on the conditions of heat and mass exchange in the crystal-melt system, which is generally described in the publication xe2x80x9cSome aspects of the macroscopic theory of oriented crystallization from the meltxe2x80x9d, E. A. Brener and V. A. Tatarchenko, Acta Physica Academias scientiarum Hungaricas, 47 (1-3) (1979) 133-138. A Stepanov crystal growth method based on a capillary formed pole of melt by means of a special mould and the crystallization of the pole outside the container is generally described in xe2x80x9cCapillary shaping in crystal growth from meltsxe2x80x9d V. A. Tatarchenko, Journal of Crystal Growth 37 (1977) 272-284 and xe2x80x9cCrystallization stability during capillary shapingxe2x80x9d G, I. Babkin, E. A. Brener and V. A. Tatarchenko, Journal Crystal Growth 50 (1980) 45-50. Another crystal growth method, the EFG crystal growth method, is a method of profiled crystal growth or a method of edge defined film fed growth. The capillary action shaping technique, CAST differs from the EFG technique in the construction of the forming mould and a presence of forced inert gas cooling. The Stepanov, EFG and CAST methods are described in the publication xe2x80x9cGrowth the profiled single crystals by Stepanov techniquexe2x80x9d P. I. Antonov, L. M. Zatulovskii, A. C. Kostyugov, (eds) Leningrad xe2x80x9cNaukaxe2x80x9d (1981) 280 pp.
The method proposed for growing a piezoelectric material based on lanthanide gallium crystals is the conventional Czochralski crystal growth method. The growth of the langasite family of crystals by the Czochralski method is accurately described in xe2x80x9cInvestigation of trigonal (La1-xNdx)3Ga2SiO14 crystalsxe2x80x9d A. A. Kaminskii, B. V. Mill, G. G. Khodzhabagyan, A. F. Konstantinova, A. I. Okorochkov, and I. M. Silvestrova, Physics Status Solid (A) 80 (1983) 387-398 and xe2x80x9cCzochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN) and La3Ga5.5Ta0.5O14 (LGT), Part I.xe2x80x9d J. Bohm, R. B. Heimann, M. Hengst, R. Roewer, J. Schindler, Journal of Crystal Growth 204 (1999) 128-136 and xe2x80x9cCzochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN) and La3Ga5.5Ta0.5O14 (LGT), Part II. Piezoelectric and elastic propetiesxe2x80x9d J. Bohm, E. Chilla, C. Flannery, H.-J. Frohlich, T. Hauke, R. B. Heimann, M. Hengst, U. Strauber, J. Schindler, Journal of Crystal Growth 216 (2000) 293-298. The Czochralski growth method of solid solutions of piezoelectric lanthanum gallium silicate single crystals is presented in the publication xe2x80x9cCzochralski growth of RE3Ga5SiO14 (RE=La, Pr, Nd) single crystals for the analysis of the influence of rare earth substitution on piezoelectricityxe2x80x9d J. Sato, H. Takeda, H. Morikoshi, K. Shimamura, P. Rudolph, T. Fukuda, Journal Crystal Growth 191 (1998) 746-753.
The successful growth of 1 or 2 inch diameter and 130 mm length La3Ga5SiO14 single crystal using the conventional Czochralski technique is described in xe2x80x9cGrowth and characterization of lanthanum gallium silicate La3Ga5SiO14 single crystals for piezoelectric applicationsxe2x80x9d Kiyoshi Shimamura, Hiroaki Takeda, Tsuguo Fukuda, Journal of Crystal Growth 163 (1996) 388-392. The Czochralski method for growth of a langasite crystal of 3-inch diameter with a 90 mm length cylindrical part along the Z-axis is described in the publications xe2x80x9cGrowth of a 3xe2x80x9d langasite crystal with clear facetingxe2x80x9d Satoshi Uda, O. Buzanov, Journal of Crystal Growth, 211 (2000) p. 318-324, xe2x80x9cGrowth of 3-inch langasite single crystal and its application to substrate for surface acoustic wave filtersxe2x80x9d, Satoshi Uda, Akihiro Bungo, Chunyun Jian, Japan Journal Applied Physics, 38 (1999) 5516-5519. Growth of langatate La3Ta0.5Ga5.5O14 crystals by the Czochralski method is described in xe2x80x9cGrowth and characterization of La3Ta0.5Ga5.5O14 single crystalsxe2x80x9d Hiroyuki Kawanaka, Hiroaki Takeda, Kiyoshi Shimamura, Tsuguo Fukuda, Journal of Crystal Growth 183 (1998) 274-277.
A method of growing single crystals of lanthanum-gallium silicate is disclosed in RU Pat. No 2126064, RU Pat. No 2143015, RU Pat. No 2126063, RU Pat. No 2108417, RU Pat. No 2108418, and W.O. Pat. No 9961686A1. The essence of the method consists in the selection of the orientation of the seed crystal ensuring growth by the Czochralski method of single crystals of lanthanum-gallium silicate along the directions  less than 01.1 greater than ,  less than 02.1 greater than ,  less than 02.3 greater than ,  less than 03.2 greater than  or at 54 degrees to the xe2x80x9cYxe2x80x9d axis. A method of growing of lanthanum gallium tantalum single crystal (LGT) is disclosed in JP Pat. No 11322495A and JP Pat. No 11199392A in which langatate crystals are doped with Pr, Nd, Ce, Sm and Eu impurities are grown by the Czochralski method.
While these methods can be used to make piezoelectric crystals, which are useful in certain applications, there is a variability of the piezoelectric behavior of different wafers cut from the same boule of lanthanide gallium crystal. The growth of langasite is characterized by distinct faceting along the (0001), (01{overscore (1)}0) and (01{overscore (1)}1) planes. Facet growth requires a greater supercooling than growth with a rough surface and hence the interface advances periodically rather than continuously.
Further, greater supercooling often leads to the presence of secondary or polycrystalline phases, resulting in scattering and/or cracking which is another problem of the conventional Czochralski method.
According to all the above-mentioned methods of growth, the lanthanide gallium melt needs to be held (soaked) at a certain temperature for a considerable time (4-20 hours). An appropriate thermal treatment of the melt is needed in order to obtain stable physical properties, which then permits further precise control of the melt temperature. An associated problem is the duration of the growth process and therefore its cost increases by up to 30%.
As described above, a bulk piezoelectric lanthanum gallium single crystal of good quality cannot presently be obtained with a high yield using a conventional Czochralski method.
It is an object of the present invention to provide a method for producing material, based on the lanthanide gallium crystals for use as piezoelectric, which displays substantially uniform piezoelectric properties throughout the crystal, a reproducibility of properties, a high yield, and resulting in a low manufacturing cost of BAW and SAW devices.
It is another object of the present invention to provide an improved Czochralski crystal growth method for producing piezoelectric material based on the lanthanide gallium crystals.
To overcome the problems described above, the present invention provides a method for producing lanthanide gallium crystals in which, to control heat and mass transfer and to maintain a steady state of crystal melt interface, the growing crystal is pulled through a forming mould dipped into the melt contained in a crucible. While a lanthanide gallium single crystal is growing, a mixed oxide charge of the same composition as the melt may be continuously added to the melt such that the quantity of melt is maintained substantially stable. Crystals produced by this method exhibit less variability in piezoelectric properties.
The shape of the solid-liquid interface is determined by the shape of the freezing isotherm. Consequently, the objects of the invention are achieved by controlling the shape of the freezing isotherm during the main part of the crystal growth. There may be some variation from the desired conditions at the beginning and the end of the growth. The thermal conditions are determined by the crucible and forming mould sizes and their location in the heating environment as well as the furnace design and the insulating material employed.
The preferred embodiments of the present invention also provide an optimal growth direction for the crystal aligned with axis perpendicular to such a crystallographic plane of lanthanide gallium crystal that an improved temperature stability, lower power flow angle, and reduced diffraction are present in a SAW device cut in this plane. Consequently, the other objects of the invention are achieved by choosing an optimum growth direction for piezoelectric applications independently of the crystallization rate. Optimal orientation allows the cutting of wafers at an angle of 90 degrees to growth axis to ensure minimal losses of material. This also means that the temperature coefficient of frequency in the wafer is close to zero. The optimally oriented lanthanum gallium crystals which are grown are more suitable for the mass-production slicing of crystals into wafers for SAW devices, suitable sizes of which may be 2 inches or more in diameter.